Piezoelectric materials are used extensively as sensors and actuators in a wide range of applications and are conventionally based on lead zirconate titanate (Pb(TiZr)O3), commonly known as PZT, especially where the ratio of lead titanate to lead zirconate is approximately 50:50, whereby the material exhibits a morphotropic phase boundary which has a mixture of tetragonal and rhombohedral symmetries. The piezoelectric property is realised after exposure of the compound to an electric field in excess of the coercive electric field which results in rotation of groups of dipoles referred to as domains. The material is then said to be “poled”. The piezoelectric effect is only apparent at temperatures less than the Curie point and, in the case of PZT, the Curie point is in the region of 300° C., thus limiting the possible applications on an operating-temperature basis. Alternative piezoelectric materials to PZT, while having the capability to withstand higher operating temperatures, suffer a significant drawback in terms of sensitivity, that is, they exhibit only a small physical change in response to drive signals.
Known alternative piezoelectric materials to PZT include BiFeO3—PbTiO3 (BFPT), as disclosed in U.S. Pat. No. 4,977,547, and BFPT doped with manganese, as disclosed in U.S. Pat. No. 4,624,796, in each case for use in underwater hydrophone devices. Although it has been thought that BFPT materials may have higher-temperature applications, especially in exhibiting a piezoelectric effect which is largely independent of temperature within the permissible operating temperature range, they do require an abnormally high coercive field which makes it extremely difficult to realise their full potential without using thin sections to minimise the risk of dielectric breakdown.
U.S. Pat. No. 6,685,849 describes a perovskite solid solution having the general formula (1−x) BiMeO3-xPbTiO3, where Me is at least one suitably-sized cation selected from scandium, indium, yttrium, ytterbium, other rare earth metals, and combinations thereof, and x is a molar fraction between about 0.5 to 0.9. The perovskite system is said to provide superior high temperature properties compared with PZT compositions. Other possible Me cations mentioned include iron, gallium, magnesium, zirconium and nickel and the compounds may optionally be doped with other elements, including lanthanum, iron, manganese, niobium, tin and barium, to optimise their properties particularly as to the Curie point. However, of the Me cations exemplified, Sc provided a maximum Curie point of only 450° C., which is still below that required for gas turbine operating environments, although In and Yb held out the prospect of higher operating temperatures.